Large aluminum electrolytic capacitors are important components in DC and AC inverter applications. Common practice is to keep equipment spares in the event of a failure. But when supporting a facility with equipment that includes banks of large aluminum electrolytic capacitors, simply having an unused spare unit is not enough to ensure rapid recovery in the event of a failure.
Periodic maintenance in the form of capacitor reforming must be performed. That way, the spare installed will be ready for immediate use.
What is Capacitor Reforming?
It’s helpful to understand that the electrolytic capacitor consists of two plates made of a thin aluminum alloy foil. The plates are separated by paper or an aluminum oxide layer to provide electrical isolation. The foil assembly is rolled up for placement in the familiar cylindrical shape packaging. All of the elements are bathed in a conductive electrolyte liquid.
One plate connection is designated the anode (+) while the other plate connection is marked as the cathode (-). The dielectric insulation between the plates comes from an aluminum oxide coating that forms on the anode foil plate. This coating occurs when a DC voltage is applied to the cathode plate during manufacturing. Reforming a capacitor refers to a process that restores the degraded aluminum oxide coating.
Under normal operation, the interaction between the electrolyte fluid and the electric field around the charged plate maintains the thickness of the aluminum oxide layer. Plate size determines the capacitance rating in Farads. The thickness of the oxide layer determines the voltage rating of the capacitor. Other parameters affect the temperature rating.
Why Do Capacitors Need to be Reformed?
Whether capacitors are stored as a component or installed as part of an equipment assembly, the result is the same. Electrolytic capacitors that remain unpowered for longer than 6 to 18 months can experience degradation of their aluminum oxide dielectric insulation. Power applied at the capacitor’s rated capacity can exploit any weakness in this insulation. This will allow the voltage to punch through to a short circuit condition.
The resulting short circuit of a capacitor results in physical and electrical damage to the capacitor. It also leads to catastrophic failure of the equipment. A severe failure of a capacitor can also cause collateral damage to surrounding equipment. For example, built up pressure inside a failed capacitor can cause physical damage in the surrounding area from capacitor material being expelled. Or, combustion damage to fragile circuitry and other components is possible. This type of failure can possibly pose a risk to personnel in the area in the event of a failure.
A rigorous schedule for reforming capacitors should be incorporated in your facility maintenance schedules. This will prevent damage to spares and intermittently idle plant systems. Manufacturers of drive and other power supply equipment that uses large electrolytic capacitors include capacitor reforming schedules in the applicable documentation.
What is the Procedure to Reform Capacitors?
The typical procedure consists of applying a small DC charge at a controlled current over a period of time. Slowly increasing the voltage level during the procedure allows the electric field to interact with the electrolyte.
This interaction will replenish the aluminum oxide layer on the anode plate. Once completed, the capacitor can be placed into immediate service. Refer to the equipment manufacturer’s documentation for more information or consult with your electrical equipment service provider.
Preventive Maintenance Saves Your Bottom Line
Keeping spare parts to replace failed equipment goes a long way toward preventing catastrophic downtime. Still, when it comes to modern capacitors, simply having them on hand isn’t enough. A regular preventive maintenance schedule that includes reforming capacitors will ensure that they are ready for immediate service.